A number of technologies have emerged to make better use of the bandwidth available on existing copper access networks. One of these technologies is xDSL. The major advantage of high-speed xDSL (Digital Subscriber Line) technologies is that they can all be supported on ordinary copper telephone cables already installed in most commercial and residential buildings. The most promising technology of the xDSL family is ADSL. Full rate ADSL provides downstream data rates of up to 8 Mbps and upstream data rates of up to 1 Mbps. A subset of full rate standard is G.Lite ADSL, which provides downstream data rates of up to 1 Mbps and upstream data rates of up to 512 Kbps depending on the telephone line condition.
The cable qualification test and ADSL modem test are two complementary tests required for deploying and troubleshooting ADSL service over existing copper lines which were originally designed for voice service. The cable qualification test is designed to verify and troubleshoot a cable used for an ADSL service by detecting and measuring any impairment of the cable. The ADSL modem test is used to verify the data rate of a cable and to troubleshoot an ADSL service with a data rate that is lower than expected. Without completing a successful ADSL modem test, service providers can not estimate the practical data rates for a potential ADSL service; a cable qualification test is not sufficient. On the other hand, a service technician can not tell from a failed ADSL modem test what caused the failure unless he or she is able to carry out a comprehensive cable qualification test.
Discrete Multi-Tone (DMT) modulation is the main technique employed by ADSL modems. It is the standard modulation adopted for use in ADSL systems by ANSI (American National Standards Institute), ETSI (European Telecom Standards Institute), and ITU (International Telecommunications Union). A pair of ADSL DMT modems will operate in Frequency Division Multiplexing (FDM) or echo cancellation mode by dividing the available frequency bandwidth into up to 256 sub-channels, or tones. Each sub-channel is modulated using QAM (Quadrature Amplitude Modulation) and carries from 0 to 15 bits/symbol/Hz. The number of bits assigned to each sub-channel is based on the measured Signal to Noise Ratio (SNR) of the cable within the sub-channel. Also, each sub-channel's data rate may be dynamically adjusted to adapt to the varying telephone line characteristics. The overall downstream and upstream data rates of the cable will be the total data rates of those sub-channels allocated for the downstream and the upstream respectively.
A general connection for an ADSL modem test of a local loop is shown in FIG. 1, where the test unit labeled as ATU-C (ADSL Termination Unit—Central) is acting as the ADSL modem at the Central Office (CO) and the other unit labeled as ATU-R (ADSL Termination Unit—Remote) is acting as the ADSL modem at the customer's premises. U-C and U-R stand for the loop interfaces at the CO site and at the remote terminal or customer's premises site. Typically, when doing a field service, one of the test units can be the ADSL device or equipment installed at the CO or at the customer's premises. In accordance with industry terminology, upstream data flows from the ATU-R to the ATU-C, while downstream data flows in the opposite direction.
The present invention provides a powerful tool to estimate the theoretical and practical data rates of a cable for various modem parameters without the need to perform different ADSL modem tests with different sets of ADSL modems. The DMT test method of this invention can also be used to analyze the causes of a problem, or to predict a problem, that a pair of ADSL DMT modems could fail to detect owing to a failure to synchronize.